1. Field of the Invention
This invention relates generally to drilling assemblies that utilize an orientation sensing system.
2. Description of the Related Art
Valuable hydrocarbon deposits, such as those containing oil and gas, are often found in subterranean formations located thousands of feet below the surface of the Earth. To recover these hydrocarbon deposits, boreholes or wellbores are drilled by rotating a drill bit attached to a drilling assembly (also referred to herein as a “bottom hole assembly” or “BHA”). Such a drilling assembly is attached to the downhole end of a tubing or drill string made up of jointed rigid pipe or a flexible tubing coiled on a reel (“coiled tubing”). For directional drilling, the drilling assembly can use a steering unit to direct the drill bit along a desired wellbore trajectory.
Wellbore drilling systems can also use measurement-while-drilling (MWD) and logging-while-drilling (LWD) devices to determine wellbore parameters and operating conditions during drilling of a well. These parameters and conditions may include formation density, gamma radiation, resistivity, acoustic properties, porosity, and so forth. Many of these tools are directionally sensitive in that, to be meaningful, the measurements made by these tools should be correlated or indexed with a frame of reference for the formation. In one convention, the angular difference between a reference point on a tool and a frame of reference such as borehole highside or magnetic north is referred to as a toolface angle. As is conventionally understood, the term “borehole highside” is an uppermost side of a non-vertical borehole. It is commonly required to present the output from imaging sensors oriented with reference to the borehole highside. Conventionally, the methodology for determining a toolface of an imaging sensor involves the use of magnetic sensing devices because the shocks, vibrations, and centrifugal forces associated with a rotating system can unduly interfere with the operation of devices such as accelerometers that could provide a direct measurement of highside. The problems encountered with such conventional devices and methods include inaccurate or outdated conversions between magnetic toolface and highside, inaccuracy due to magnetic junk or hotspots, eddy currents induced in a rotating conductive collar, and errors caused by electric currents flowing in proximity to the sensor. In addition, while it is desirable to continuously measure the azimuth of the borehole while drilling, the value of such measurements has been limited due to the difficulty of accurately measuring transverse acceleration components of a rotating system.
The present invention is directed to addressing one or more of the above stated drawbacks for determining the orientation of logging tools and other components of a drilling system.